Ripple cancellation converter with high power factor

ABSTRACT

Provided are circuits and methods for use with a power supply that provides a main output including a main DC voltage having a first AC voltage ripple, or a main DC current having a first AC current ripple. A ripple cancellation converter provides a second AC voltage ripple connected in series with the main output, such that the first AC voltage ripple is substantially cancelled; or a second AC current ripple connected in parallel with the main output, such that the first AC current ripple is substantially cancelled. As a result, substantially ripple-free DC output power is provided.

RELATED APPLICATIONS

This application is a 371 of International Application No.PCT/CA2012/000971, filed Oct. 17, 2012, which claims the benefit of thefiling date of U.S. Application No. 61/547,926, filed Oct. 17, 2011, thecontents of all of which are incorporated herein by reference in theirentirety.

FIELD

This invention relates to methods and circuits for improving theperformance of an AC to DC power supply. In particular, methods andcircuits are provided for power supplies that achieve high power factor(PF) at the AC side and at the same time, reduce or substantiallyeliminate ripple in the DC output power. The methods and circuits may beused in any application where high power factor and/or low output rippleare required. In particular, the methods and circuits may be used in DClighting applications, such as in light emitting diode (LED) lighting,wherein suppression of low frequency ripple in the output powereliminates visible flickering.

BACKGROUND

Regulation concerning power factor correction for a wide range ofdevices is becoming increasingly stringent. For example, new regulationrequires power factor correction for any light emitting diode (LED)power supply with output power higher than 5 W.

For low to medium power level (e.g., 5 W to 100 W), a flyback converteris often used. FIG. 1 shows a block diagram of such a system. By forcingthe average input current to follow the input voltage, high power factorcan be achieved. In order to reduce the cost, critical conduction modeis often used to achieve power factor correction. However, this resultsin a ripple in the output voltage at harmonics of the line frequency.The second harmonic (e.g., 120 Hz for North America or 100 Hz for China,Europe) is of particular concern for DC lighting applications, such asLED lighting, as it results in visible flickering wherein the human eyecan see fluctuation of the light emitting from the LED.

In such a conventional design there is a compromise between power factorand small low frequency current ripple through the load. For example, inorder to achieve a power factor higher than 90%, the peak to peak valueof the load current ripple can be as high as 60% of the average DCvalue. For example, for an average load current of 0.5 A, the lowfrequency current ripple can be as high as 0.3 A (peak to peak). Thisraises several problems: Firstly, as mentioned above, for DC lighting(e.g., LED) applications, the ripple current causes visible flickering.Secondly, it is difficult to achieve variable output power. When theaverage load current is reduced, the ripple current does not decreaseproportionally and therefore the ripple current will become more of aproblem at reduced output power. In DC lighting applications, flickeringwill be worse at reduced brightness. Thirdly, the ripple currentdegrades the lifespan of many devices, such as LEDs.

To achieve high power factor and small low frequency current ripple,two-stage conversion may be used. FIG. 2 shows a circuit block diagramof a conventional converter used to drive an LED, where the first stage20 is typically a boost converter that converts the AC voltage into ahigh voltage, e.g., 400 V, and also achieves power factor correction.The second stage 22 is a DC to DC converter that converts the 400 V intoa lower voltage required by the load 100, e.g., 50 V or 125 V, provideselectrical isolation, and regulates the load current. However, comparedto the converter of FIG. 1, the converter of FIG. 2 suffers from thedrawbacks of higher cost and larger size.

SUMMARY

Provided herein is a circuit for use with a power supply that provides amain output including a main DC voltage with a first AC voltage rippleor a main DC current with a first AC current ripple, the circuitcomprising a ripple cancellation converter that: (i) provides a secondAC voltage ripple and is connected in series with the main output, suchthat the first AC voltage ripple is substantially cancelled; or (ii)provides a second AC current ripple and is connected in parallel withthe main output, such that the first AC current ripple is substantiallycancelled; wherein substantially ripple-free DC output power isprovided.

Also provided herein is a circuit that provides DC power, comprising: aportion that outputs a main DC voltage with a first AC voltage ripple ora main DC current with a first AC current ripple; and a ripplecancellation converter that: (i) provides a second AC voltage ripple andis connected in series with the main output, such that the first ACvoltage ripple is substantially cancelled; or (ii) provides a second ACcurrent ripple and is connected in parallel with the main output, suchthat the first AC current ripple is substantially cancelled; whereinsubstantially ripple-free DC output power is provided.

In some embodiments, the ripple cancellation converter provides anauxiliary output comprising: (i) an auxiliary DC voltage with the secondAC voltage ripple, wherein the second AC voltage ripple is substantiallyequal in magnitude and substantially opposite in phase to the first ACvoltage ripple; or (ii) an auxiliary DC current with the second ACcurrent ripple, wherein the second AC current ripple is substantiallyequal in magnitude and substantially opposite in phase to the first ACcurrent ripple; wherein the main output and the auxiliary output arecombined such that: (i) a total DC voltage or a total DC current isprovided; (ii) the second AC voltage ripple substantially cancels thefirst AC voltage ripple, or the second AC current ripple substantiallycancels the first AC current ripple; (iii) the total DC voltage or thetotal DC current is substantially ripple-free. The ripple cancellationconverter may comprise a buck converter, a boost converter, a buck-boostconverter, or a full bridge converter.

In one embodiment, the circuit further comprises a sensor that sensesthe first AC voltage ripple or the first AC current ripple, and outputsa sensor signal to the ripple cancellation converter. In one embodiment,the circuit further comprises a power factor correction portion.Operation of the power factor correction portion may be based on afeedback signal derived from a load current.

In some embodiments, the substantially ripple-free DC output power isvariable.

Embodiments may be used with an AC-DC power supply. In one embodiment,the portion that outputs a main DC voltage with a first AC voltageripple or a main DC current with a first AC current ripple comprises anAC-DC power supply. The AC-DC power supply may comprise a flybackconverter, an isolated boost converter, a buck-boost converter, a buckconverter, or a boost converter.

In some embodiments, a load comprises an LED.

Also provided herein is a method for providing DC power from a mainoutput power comprising a main DC voltage with a first AC voltage rippleor a main DC current with a first AC current ripple, the methodcomprising: (i) connecting a second AC voltage ripple in series with themain output power, such that the first AC voltage ripple issubstantially cancelled; or (ii) connecting a second AC current ripplein parallel with the main output, such that the first AC current rippleis substantially cancelled; wherein substantially ripple-free DC outputpower is provided.

Also provided herein is a method for providing DC power, comprising:outputting a main output power comprising a main DC voltage with a firstAC voltage ripple or a main DC current with a first AC current ripple;and (i) connecting a second AC voltage ripple in series with the mainoutput power, such that the first AC voltage ripple is substantiallycancelled; or (ii) connecting a second AC current ripple in parallelwith the main output, such that the first AC current ripple issubstantially cancelled; wherein substantially ripple-free DC outputpower is provided.

In some embodiments the method further comprises: (i) providing anauxiliary DC voltage with the second AC voltage ripple, wherein thesecond AC voltage ripple is substantially equal in magnitude andsubstantially opposite in phase to the first AC voltage ripple; or (ii)providing an auxiliary DC current with the second AC current ripple,wherein the second AC current ripple is substantially equal in magnitudeand substantially opposite in phase to the first AC current ripple;wherein connecting includes combining the auxiliary DC voltage with themain DC voltage, or combining the main DC current with the auxiliary DCcurrent, such that: (i) a total DC voltage or a total DC current isprovided; (ii) the second AC voltage ripple substantially cancels thefirst AC voltage ripple, or the second AC current ripple substantiallycancels the first AC current ripple; (iii) the total DC voltage or thetotal DC current is substantially ripple-free.

Embodiments may include using a buck converter, a boost converter, abuck-boost converter, or a full bridge converter to provide the secondAC voltage ripple or to provide the second AC current ripple.

One embodiment further comprises sensing the first AC voltage ripple orthe first AC current ripple, and outputting a sensor signal to theripple cancellation converter.

Another embodiment further comprises adjusting a power factor of acircuit used to provide DC power. The method may comprise adjusting thepower factor based on a feedback signal derived from a load current.

Embodiments may include varying the substantially ripple-free DC outputpower.

Embodiments may be used with an AC-DC power supply. Various embodimentsinclude using a flyback converter, an isolated boost converter, abuck-boost converter, a buck converter, or a boost converter for theAC-DC power supply.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, and to show more clearlyhow it may be carried into effect, embodiments will be described, by wayof example, with reference to the accompanying drawings, wherein:

FIG. 1 is a block diagram of an LED driver with a flyback converteraccording to the prior art;

FIG. 2 is a block diagram of a two-stage LED driver according to theprior art;

FIG. 3 is a block diagram of a power supply with a ripple cancellationcontroller according to one embodiment;

FIG. 4 is a block diagram of a power supply with a ripple cancellationcontroller and control architecture, according to one embodiment;

FIG. 5 is a plot showing voltage waveforms of the embodiment of FIG. 4;

FIG. 6 is a block diagram of a power supply with a ripple cancellationcontroller and load current control according to one embodiment;

FIG. 7 is a block diagram of a power supply with a ripple cancellationcontroller based on an isolated boost converter, according to oneembodiment;

FIG. 8 is a diagram of power supply with a ripple cancellationcontroller based on a non-isolated buck-boost converter that generatesoutput Vo1, according to one embodiment;

FIG. 9 is a schematic diagram of power supply with a ripple cancellationcontroller based on a full bridge converter and pulse width modulationcontrol, according to one embodiment;

FIG. 10 is a schematic diagram of a ripple cancellation converterimplemented with a buck converter, according to one embodiment;

FIG. 11(a) is a schematic diagram of a ripple cancellation converterimplemented with a buck-boost converter, according to one embodiment;

FIG. 11(b) is a diagram of a power supply with current control using asecond primary winding for isolation, according to one embodiment;

FIG. 12 is a block diagram of a power supply with current control and anexternal DC reference voltage, according to one embodiment;

FIG. 13 is a block diagram of driver power supply with aparallel-connected ripple current cancellation controller and loadcurrent control, according to one embodiment;

FIG. 14 is a schematic diagram of driver power supply secondary side andcontrol circuitry suitable for integrated circuit implementation,according to one embodiment;

FIG. 15(a) is a schematic diagram of driver power supply secondary sideand control circuitry suitable for integrated circuit implementation,according to another embodiment;

FIG. 15(b) is a schematic diagram of driver power supply secondary sideand control circuitry suitable for integrated circuit implementationbased on the embodiment of FIG. 14(a), wherein an opto-coupler is notused;

FIG. 16 is a schematic diagram of driver power supply secondary side andcontrol circuitry suitable for integrated circuit implementation,wherein load current control is achieved by applying a voltage to acurrent amplifier;

FIG. 17 is a schematic diagram of driver power supply secondary side andcontrol circuitry based on a buck-boost converter, suitable forintegrated circuit implementation, according to another embodiment;

FIG. 18 is a schematic diagram of driver power supply secondary sidesuitable for implementation using discrete components, according to oneembodiment;

FIG. 19 is a block diagram showing driver power supply secondary side atleast partially implemented in an integrated circuit, wherein the DCreference voltage is inside the integrated circuit, according to oneembodiment;

FIG. 20 is a block diagram of an embodiment with variable output power,wherein LED dimming control is provided by using an on/off controlsignal for the ripple cancellation converter;

FIG. 21 is a schematic diagram of a power supply with variable outputpower wherein a buck converter is used for the ripple cancellationconverter, according to one embodiment;

FIG. 22 is a plot showing a control signal (top) and load current(bottom) for the embodiment of FIG. 21;

FIG. 23 is a block diagram of a test setup used to evaluate performanceof a power supply with a ripple cancellation converter according to oneembodiment;

FIG. 24 is a plot showing performance for the test setup of FIG. 23,wherein the upper waveform is the DC coupled load current (200 mA/div),the middle waveform is the AC coupled load current (10 mA/div), and thebottom waveform is the AC ripple voltage at the main output, betweenVo1+ and Vo1− (2 V/div); and

FIG. 25 is a plot of the AC input current waveform at 110 V AC and 50 V,0.7 A output, for the test setup of FIG. 23.

DETAILED DESCRIPTION

Provided herein are AC to DC power supply methods and circuits thatprovide substantially ripple-free output power and achieve high powerfactor. The methods and circuits provided herein may be used in anypower supply application where substantially ripple-free output powerand high power factor are desirable, such as, but not limited to,computers, tablets, cell phones, etc. Embodiments are describedprimarily with respect to lighting applications, e.g., wherein the loadis an LED; however, it is to be understood that they are not limitedthereto.

For the sake of brevity, the term “LED” as used herein is intended torefer to a single LED or to multiple LEDs electrically connectedtogether (e.g., in a series, parallel, or series-parallel arrangement).It will be appreciated that an LED lighting fixture (e.g., an LED lamp)may include a single LED or multiple LEDs electrically connectedtogether.

The methods and circuits achieve high power factor (PF) at the AC sideand reduce or substantially eliminate ripple in the DC output power. Theripple may be a harmonic of the line frequency, such as the secondharmonic (e.g., 120 Hz for North America or 100 Hz for China, Europe) ofthe line frequency. In DC lighting (e.g., LED) applications, ripple atthe 2^(nd) harmonic results in visible flickering, wherein the human eyecan see fluctuation of the light emitting from the LED. Therefore, ofparticular interest for lighting applications is suppression orelimination of ripple at the 2^(nd) harmonic. The methods and circuitsdescribed herein reduce or substantially eliminate ripple at the secondharmonic and at other harmonics. The methods and circuits providedherein minimize component counts, providing power supplies that arecompact and cost effective. Embodiments may be implemented in anycurrently-available semiconductor technology. Embodiments may beimplemented as a hybrid circuit comprising one or more integratedcircuit (IC) component together with discrete components, orsubstantially as an IC.

Provided herein are circuits and methods for use with a power supplythat provides a main output including a main DC voltage having a firstAC voltage ripple, or a main DC current having a first AC currentripple. The methods and circuits include a ripple cancellation converter(RCC). The ripple cancellation converter (i) provides a second ACvoltage ripple connected in series with the main output, such that thefirst AC voltage ripple is substantially cancelled; or (ii) provides asecond AC current ripple connected in parallel with the main output,such that the first AC current ripple is substantially cancelled. As aresult, substantially ripple-free DC output power is provided.

In some embodiments, a first portion provides most of the DC power(i.e., the main output power) to the load with a small AC component(i.e., a selected amount of voltage ripple or current ripple), and asecond portion provides a small amount of DC power (i.e., auxiliarypower) to the load and an AC component that cancels the voltage rippleor current ripple produced by the first portion. The AC componentprovided by the second portion is substantially equal in magnitude andsubstantially opposite in phase (i.e., inverse) to the AC componentprovided by the first portion, such that combining the AC components ofthe first and second portions substantially cancels ripple in the totaloutput power to the load. The total output power to the load thereforeincludes the main DC power provided by the first portion together withthe auxiliary DC power provided by the second portion, substantiallywithout ripple. The first portion may be implemented with fewer and/orsmaller components, such as, for example, a smaller output capacitor,such that size and cost of the first portion is reduced, and efficiencyis improved. Also, since the auxiliary power provided by the secondportion is only a small amount of the total output power, it may beimplemented using passive components with smaller inductance andcapacitance values, and active components with lower voltage and/orcurrent ratings, so as to achieve ripple cancellation and supply therequired auxiliary output power with reduced cost and size relative to aconventional converter such as shown in FIG. 2. Power supply methods andcircuits provided herein include series-connected embodiments forvoltage ripple cancellation, wherein the output of a ripple cancellationconverter is connected in series with a main output, andparallel-connected embodiments for current ripple cancellation, whereinthe output of a ripple cancellation converter is connected in parallelwith a main output.

Series Ripple Cancellation Converter

A series-connected embodiment based on a flyback converter will now bedescribed with reference to FIG. 3. In the embodiment of FIG. 3, theprimary side includes a full-wave bridge rectifier and a switch S inseries with a transformer primary winding Np. A power factor correction(PFC) controller controls the switch S to achieve high power factor atthe primary (input) side. Different from the conventional flybackarchitecture shown in FIG. 1, the secondary side includes first andsecond secondary windings Ns1 and Ns2, respectively. Winding Ns1provides the main voltage Vo1. Winding Ns2 provides a secondary voltageVo2. The voltage Vo2 is followed by a ripple cancellation converter 35with auxiliary output voltage Vo3. The main voltage Vo1 and theauxiliary voltage Vo3 are connected in series to provide the totaloutput voltage Vout for the load 100.

When power factor correction is achieved, or when the power factor ishigh, the output voltage Vo1 will contain a significant amount of lowfrequency voltage ripple at a frequency two times the line frequency(e.g., 120 Hz in North America), as shown in the following equation:Vo1=Vo1_dc+Vo1_rip  (1)

In the above equation, Vo1_dc represents the DC component of Vo1 andVo1_rip represents the low frequency component of Vo1.

The output of the ripple cancellation converter may be expressed by thefollowing equation:Vo3=Vo3_dc+Vo3_rip  (2)

In the above equation, Vo3_dc represents the DC component of Vo3, andVo3_rip represents the low frequency component of Vo3.

Therefore, the total output voltage which will be applied to the loadmay be derived as:Vout=Vo1+Vo3=Vo1_dc+Vo3_dc+Vo1_rip+Vo3_rip  (3)

The DC value (Vout_dc) and low frequency ripple value (Vout_rip) of thetotal output Vout may be expressed as:Vout_dc=Vo1_dc+Vo3_dc  (4)Vout_rip=Vo1_rip+Vo3_rip  (5)

The ripple cancellation converter is constructed and controlled suchthat the low frequency voltage ripple (Vo3_rip) is the same in magnitudeand inverse in phase compared with the low frequency voltage ripple inVo1, as shown in the following equation:Vo3_rip=−Vo1_rip  (6)

Substituting (6) into (5), the following equation is obtained:Vout_rip=0  (7)

Therefore, the voltage across the load will contain substantially no lowfrequency ripple. In the case where the load is an LED, flickering ofthe LED is avoided and lifespan of the LED is not degraded.

FIG. 4 shows a block diagram of a series embodiment with control blocks.In the figure, a Vo1 ripple sensor 40 senses the low frequency ripplecomponent of Vo1 and provides an output of the ripple with inversepolarity to a RCC control circuit 37. The Vo1 ripple sensor may include,for example, a high pass filter. The Vo1 ripple sensor output is−Vo1_rip. Vo3_dc_ref is a reference voltage for the DC value of the RCCoutput voltage. The reference of the RCC isVo3_ref=Vo3_dc_ref+(−Vo1_rip). Therefore, the control circuit 37 willregulate the output voltage Vo3 as:Vo3=Vo3_dc_ref+(−Vo1_rip)=Vo3_dc_ref−Vo1_rip  (8)

Therefore, the output voltage Vout may be expressed as:Vout=Vo1_dc+Vo1_rip+Vo3_dc_ref−Vo1_rip=Vo1_dc+Vo3_dc  (9)

In the above equation, Vo3_dc=Vo3_dc_ref. The value of Vo1_dc may beregulated by the PFC controller 30 to control the DC value of the loadcurrent. This is discussed in detail below.

Whereas it may be desirable to reduce the power consumption, andtherefore size, of the ripple cancellation converter, the DC value ofVo3 (Vo3_dc) should be minimized. For example, the value of Vo3_dc_refmay be set to 0, in which case the output voltage of the ripplecancellation converter is only the ripple voltage of Vo1, Vo1_rip.

FIG. 5 shows the waveforms of Vo1 (top), Vo3 (middle) and Vout (bottom)when only low frequency ripple is considered. It can be observed thatthe low frequency voltage ripple in Vo1 has been completely compensatedby Vo3 and the output voltage (Vout) that will be applied to the loaddoes not contain any low frequency ripple.

In other embodiments the DC value of Vo3 (Vo3_dc) is minimized, but isgreater than zero so as to provide a DC offset that is high enough toprevent the instantaneous output voltage from going below 0 V. That is,the value of Vo3_dc_ref may be set to a value equal to or higher thanthe peak value of the ripple. For example, if the peak value of theripple is 1.5 V, Vo3_dc_ref may be set to 2 V, in which case the outputvoltage of the ripple cancellation converter is the ripple voltage ofVo1, Vo1_rip with DC offset Vo3_dc. To minimize power dissipation in theRCC, Vo3_dc_ref may be set to a value equal to or slightly higher thanthe peak value of the ripple.

The series topology and control strategy embodiments described aboveremove or substantially reduce ripple, such as low frequency ripple(e.g., 100 Hz or 120 Hz), in the DC output power. In the power supplyembodiments described herein, ripple cancellation and power factorcorrection are achieved at the same time by separate control loops.Ripple cancellation is achieved by the ripple cancellation converter 35and its related control circuit 37. Power factor correction is achievedby the PFC controller 30 at the primary side.

Load Current Control

The load current may be controlled by regulating the DC value of Vo1.FIG. 6 shows the embodiment of FIG. 4 with current control added toachieve this objective. Operation of this embodiment will be describedfor the case where the load 100 is an LED. The LED current is sensedusing a current sense resistor Rs. The sensed signal V_I_led is comparedwith a reference LED current I_ref by a current error amplifier 60, inthis case OpAmp1. Its output is the error voltage Verror_s, which istransferred to the primary side using an isolating device 62 such as,for example, an opto-coupler. Verror_p is used to control the PFCcontroller on the primary side to adjust the voltage level of Vo1. Bychanging the current reference level I_ref, the LED current may becontrolled by the PFC controller. The ripple voltage across Vo1 will beautomatically compensated by the ripple cancellation converter 35.Therefore, power factor control and LED current control are de-coupledand high power factor can be achieved over wide LED current variationrange.

In the above embodiments a flyback converter is used as an example of animplementation for power factor correction. It will be understood thatother converters may also be used. For example, a boost converter withisolating transformer for power factor correction may also be used, asshown in FIG. 7.

Further, the output Vo3 may also be generated using other converterdesigns. For example, in the embodiment shown in FIG. 8, the output Vo1is generated with a non-isolated buck-boost converter. The ripplecancellation converter, which generates Vo3, may be implemented using aflyback converter. In this configuration, the voltage ripple in Vo1 maybe compensated by Vo3 using the above-mentioned control method. The DCvalue of the load current is controlled by the DC value of Vo1 (Vo1_dc).Vo1_dc is controlled by the PFC controller of the buck-boost converter.

Ripple Cancellation Converter Implementation

The ripple cancellation converter may be implemented with differenttypes of switching converters. One example is a full bridge converter,as shown in FIG. 9. In this embodiment, the DC value of the full bridgeconverter may be zero, a positive value, or a negative value. Its lowfrequency ripple may be controlled to be the same as the low frequencyripple of Vo1 with inverse polarity. That is, the output of the fullbridge converter may be regulated as:Vo3=Vo3_dc−Vo1_rip  (10)

For embodiments where a buck converter is used as the ripplecancellation converter, an example of a power circuit is shown in FIG.10. In FIG. 10, only the secondary side circuit is shown (the primaryside circuit is the same as in FIG. 9). Using a buck converter, the DCvalue of Vo3 cannot be reduced to zero. Similarly, −Vo1_rip is used aspart of the reference voltage for the buck converter. The output voltageVo3 is controlled as:Vo3=Vo3_dc−Vo1_rip  (11)

The maximum and minimum value of Vo3 may be calculated as:Vo3_max=Vo3_dc+0.5*Vo1_rip_pp  (12)Vo3_min=Vo3_dc−0.5*Vo1_rip_pp  (13)

In the above equations, Vo1_rip_pp represents the peak to peak value ofthe low frequency ripple for Vo1. It is advantageous to set the Vo3_dcto be a little bit higher than 0.5*Vo1_rip_pp. In this way, the powerprovided by the buck converter will be minimized and the overallefficiency will be improved.

In addition, in order for the buck converter to operate properly, thetransformer secondary side turns ratio (Ns1 and Ns2), as well as theoutput capacitor values (C1 and C2) should be selected so that thefollowing relations are satisfied:Vo2_min>Vo3_dc+0.5*Vo1_rip_pp  (14)Vo2_min=Vo2_dc−0.5*Vo2_rip_pp  (15)

In equation (15), Vo2_rip_pp represents the peak to peak value of thelow frequency ripple for Vo2. That is, the minimum Vo2 value should behigher than the maximum value of Vo3.

The power circuit for an embodiment where a buck-boost converter is usedas the ripple cancellation converter is shown in FIG. 11(a). In FIG.11(a), only the secondary side is shown (the primary side circuit is thesame as in FIG. 9). Of note is the reference direction of the voltageVo2.

As described above, −Vo1_rip is used as part of the reference voltagefor the buck-boost converter. The output voltage Vo3 may be controlledas:Vo3=Vo3_dc−Vo1_rip  (16)

In order to achieve ripple cancellation, Ns1, Ns2 and C1, C2 should beselected properly to meet the following requirement:Vo3_dc>Vo3_min+0.5*Vo1_rip_pp  (17)

That is, the minimum output voltage of Vo3 should be higher than zero.The ripple cancellation converter may also be implemented using otherswitching converters, such as, but not limited to, a boost converter, aswould be apparent to one of ordinary skill in the art.

In practice, it is desirable to limit the DC value of Vo3 (Vo3_dc) tojust slightly higher than half the ripple voltage, (0.5*Vo1_rip_pp).Therefore, a control strategy according to one embodiment is to sensethe peak to peak value of the ripple voltage (Vo1_rip_pp) and then setthe DC value of Vo3 asVo3_dc_ref=0.5*Vo1_rip_pp+V_offset  (17a)

In equation (17a), V_offset is a small DC value, such as, for example,0.5V, 0.75V, 1.0V, etc. Alternatively, Vo3_dc_ref may be determined bythe following equation:Vo3_dc_ref=(0.5+K_offset)*Vo1_rip_pp  (17b)

In equation (17b), K_offset is a small positive value, such as, forexample, 0.1 to 1. With this arrangement, the relation (17) will alwaysbe satisfied under different ripple conditions for Vo1.

As described above, an isolating device such as an opto-coupler may beused where the load current is regulated by a primary side PFCcontroller. In some applications where an opto-coupler cannot be used orwhere it is not preferred to use an opto-coupler, the load current mayalso be regulated using the DC value of Vo3.

As can be observed from equation (9), when Vo3_dc is changed, thevoltage across the load will also change and, consequently, the loadcurrent will change. In this case, the voltage Vo1 will be controlled tobe substantially constant through primary side voltage sensing bycontrolling a primary auxiliary voltage at the primary side as describedbelow with reference to FIG. 11(b).

In FIG. 11(b), a second primary winding Np2 is added at the primaryside. Diode D_aux and capacitor C_aux are used to create an auxiliaryvoltage for the PFC controller. Because of the nature of a flybackconverter, the relationship between V_aux and Vo1 can be expressedapproximately as the following:V_aux=Vo1*Np2/Ns1  (17c)

It can be observed that by maintaining V_aux at a predetermined level,Vo1 can also be controlled. Therefore, the output voltage Vout can becontrolled by changing Vo3. At the secondary side, the voltage acrossthe current sense resistor Rs is sent to the ripple cancellationconverter, which changes the voltage Vo3 to regulate the load current.

In FIG. 11(b), Rc1 and Rc2 are used to detect the zero crossing of themagnetizing inductor current. Of course, other methods and circuits maybe used to control operation of the PFC, as will be readily apparent toone of ordinary skill in the art.

The block diagram of FIG. 12 shows an embodiment with an LED loadwherein an external DC reference voltage is used. In this embodiment,the ripple voltage of the main output voltage (between Vo1+ and Vo1−) issensed at 120 and separated at 122 from its DC component, and added atsummer 124 to the DC reference voltage of the ripple cancellationconverter 35, and the combined voltage (Vrcc_ref) is used as thereference voltage for the ripple cancellation converter.

Parallel Ripple Cancellation Converter

The above embodiments provide series-connected ripple cancellationconverters, wherein the main output voltage Vo1 and the auxiliary outputvoltage Vo3 from the ripple cancellation converter are connected inseries and added such that the ripple voltage in the main output iscancelled. However, parallel-connected embodiments are also providedherein. In parallel compensation, the auxiliary output of a currentripple cancellation converter 35 is connected in parallel with the mainoutput, as shown in the embodiment of FIG. 13.

In FIG. 13, the total current for the load 100, which may be an LED asshown, is I_led. The main output Vo1 provides most of the current I_o1required for the LED. This current contains low frequency ripple (e.g.,at either 100 Hz or 120 Hz). The output current of the main output canbe described by the following equation:I_o1=I_o1_dc+I_o1_rip  (18)

wherein I_o1_dc represents the DC value and I_o1_rip indicates the lowfrequency ripple of the main output current.

The output current I_o1 is sensed and its ripple component I_o1_rip isretrieved by an I_o1 ripple sensor circuit 130.

The output current of the current ripple cancellation converter 35 iscontrolled such that:

$\begin{matrix}\begin{matrix}{{{I\_ o}\; 2} = {{{I\_ o2}{\_ dc}} - {{I\_ o1}{\_ rip}}}} \\{= {{{I\_ o2}{\_ dc}{\_ ref}} - {{I\_ o}\; 1{\_ rip}}}}\end{matrix} & (19)\end{matrix}$

That is, the output current of the current ripple cancellation converterhas a DC value determined by a reference (I_o2_dc_ref) and an AC valueequal to the inverse of the ripple value of I_o1. Therefore, the currentthrough the LED load will be:

$\begin{matrix}\begin{matrix}{{I\_ led} = {{I\_ o1} + {I\_ o2}}} \\{= {{{I\_ o1}{\_ dc}} + {{I\_ o1}{\_ rip}} + {{I\_ o2}{\_ dc}} - {{I\_ o1}{\_ rip}}}} \\{= {{{I\_ o1}{\_ dc}} + {{I\_ o2}{\_ dc}}}}\end{matrix} & (20)\end{matrix}$

From equation (20) it can be seen that there is no ripple component forthe LED current.

To improve efficiency, I_o2_dc may be selected to be smaller thanI_o1_dc.

The LED current is controlled by an LED current controller 60 andisolating device 62 such as an opto-coupler, and the error signal issent to the primary side to provide a reference for a power factorcorrection controller. By adjusting the LED current reference I_led_ref,the output voltage of Vo1 changes and the DC value of the main outputcurrent I_o1 (I_o1_dc) also changes, such that the LED current changes.

In other embodiments, other converter types may be used to implement thecurrent ripple cancellation converter. For example, in one embodiment abuck converter is used. In this case, the voltage Vo2 should be higherthan Vo3, which is substantially the same as Vo1, as they are connectedtogether in parallel.

In another embodiment, a boost converter is used to implement thecurrent ripple cancellation converter. In this case, Vo2 should be lowerthan Vo3.

In another embodiment, a buck-boost converter is used to implement thecurrent ripple cancellation converter. In this case, the polarity of theinput voltage Vo2 of the current ripple cancellation converter isreversed relative to the polarity of Vo1.

Integrated Circuit Implementation

In some applications, such as an LED driver, the current for the ripplecancellation converter is not large, for example, it may be in the rangeof 0.5 A to 1 A, depending on the application. The ripple voltage isnormally less than 10 V (peak to peak value). For some applications, apreferred way to implement the ripple cancellation converter and therelated control circuit may be in an integrated circuit (IC) chip. Forexample, the IC chip may include the switch (e.g., MOSFET) and controlcircuits. For example, in one embodiment the high side switch (such asSb1 in FIG. 10) may be implemented with a P channel MOSFET toconveniently facilitate gate drive. This is described in greater detailbelow, wherein, for example, a buck converter is used as the ripplecancellation converter. Other converter types may also be used, as wouldbe apparent to one of ordinary skill in the art.

FIG. 14 shows a circuit diagram of a ripple cancellation converter andrelated control circuits according to one embodiment, wherein the loadis an LED. The secondary side ground is at the negative terminal of Vo2.R1 and R2 form a resistor divider from the positive terminal of Vo1(Vo1+) to ground. Cd1 and Rd1 form a DC blocking circuit that blocks theDC component in Vo1+. Similarly, R3 and R4 is a resistor divider fromthe negative terminal of Vo1 (Vo1−) to ground. Cd2 and Rd2 is a DCblocking circuit that blocks the DC component of Vo1−. The signalsVo1−_s and Vo1+_s are fed to a circuit 140, in this case OpAmp1 whichcalculates the difference and reconstructs the ripple voltage of Vo1.The output of OpAmp1 is offset by a voltage with value of Vo3_dc_ref.Therefore, the reference voltage for the buck converter is:Vo3_ref=Vo3_dc_ref−(Vo1+_s−Vo1−_s)  (21)

The value of Vo3_dc_ref is set to be slightly higher than0.5*(Vo1+_s−Vo1−_s), but not substantially higher, so that the energyprovided by the buck converter is minimized.

The output voltage Vo3 is sent to the IC through resistor divider R5 andR6 and compared with Vo3_refusing a voltage error amplifier 142, in thiscase OpAmp2. Its output is sent to the pulse width modulator (PWM) anddriver 144 to control the duty cycle of the buck switches Sb1 and Sb2.

In the design, the ratios R1/R2, R3/R4, and R5/R6 should besubstantially the same in order to substantially compensate (i.e.,cancel) the low frequency voltage ripple in Vo1.

The LED current is sensed by the resistor Rs and sent to the currenterror amplifier 60, OpAmp3. The LED current reference signal may begenerated inside the IC. The output of the current error amplifier(I_error_s) is sent to the primary side using an opto-coupler(I_error_p) and then used to control the PFC circuit to regulate Vo1.

The pin assignments (pin1, pin2, pin3, . . . , pin9) indicate possiblepin out numbers for the integrated circuit. For example, in theembodiment shown, a 9 pin IC chip is needed. In this example Pin8 is theground or reference point of the integrated circuit chip. It is also thereference point for the secondary side.

In a practical design, it is preferred to have an 8 pin IC chip in orderto reduce cost. FIG. 15(a) shows an embodiment where the ripplecancellation converter and control circuits are implemented with an 8pin IC chip. It is noted that from a power flow point of view, there isno difference between the embodiment shown in FIG. 14 and the embodimentshown in FIG. 15(a).

The main difference between the embodiments in FIG. 14 and FIG. 15(a) isthat in FIG. 15(a), Vo3 is connected on top of Vo1 and the secondaryside reference point (ground) is moved to the negative terminal of Vo3(Vo3−). The positive terminal of Vo1 (Vo1+) is connected to the negativeterminal of Vo3 (Vo3−) through the current sense resistor Rs. Therefore,Vo1+ is almost zero. The low frequency ripple voltage of Vo1 is obtainedby resistor divider R1, R2, Cd1, and Rd1, as indicated by Vo1−_s.Vo1−_s=−Vo1_rip*R2/(R1+R2)  (22)

Vo1−_s is sent to the IC chip through pin 4. In the IC chip, Vo1−_s isadded with a voltage Vo3_dc_ref and the combined voltage is used as thereference voltage for Vo3. It is noted that the ratio of R1/R2 and R5/R6should be substantially the same in order to substantially compensate(i.e., cancel) the low frequency voltage ripple in Vo1. That is, thecloser the ratios of R1/R2 and R5/R6, the better will be thecompensation.

If an opto-coupler is not used, the output of OpAmp3 (as shown in FIG.15(a)) will be sent to the positive input terminal of OpAmp2 to changethe DC reference voltage for Vo3. The embodiment of FIG. 15(b) shows onesuch implementation. The output of OpAmp3 is connected to Vo3_refthrough resistor R20.

In some embodiments, the load current is controlled from the primaryside. In this case, the current error signal, I_error_s is sent to theprimary side. For example, the error signal 1_error_s may be sent to theprimary side using an isolating device 62, such as, for example, anopto-coupler, as shown in FIGS. 14 and 16. In FIGS. 14 and 16, the load100 is shown as an LED. It is noted that the load current may beadjusted by applying a voltage to the current amplifier as shown in theembodiment of FIG. 16. A voltage (I_led_con) may be applied to signal1_led (pin6) using resistors R11 and R12. In this way, the voltageapplied to the negative terminal of OpAmp3 may be adjusted.

The embodiment of FIG. 17 shows an IC implementation with a buck-boostconverter used as the ripple cancellation converter. This embodiment maybe implemented with an 8 pin IC package. In this case, the Vcc for theIC chip is the sum of Vo2 and Vo3. It is noted that because ofrelatively larger low frequency ripple across C3, the Vcc of the IC chipwill also contain several volts of ripple. A linear regulator may beincorporated inside the IC to compensate for the ripple voltage.

One advantage of using a buck-boost converter is that it permits a widerVo3 voltage range, as Vo3 may be higher or lower than its input voltageVo2.

The embodiment of FIG. 18 shows another implementation where low cost,off the shelf components (i.e., discrete components) may be used for theripple cancellation converter, and to achieve low current ripple throughthe load and high power factor at the AC side. In this embodiment, thediode D3 of the buck-boost converter is moved to the top side. Thesecondary side ground is the negative terminal of Vo3 and the positiveterminal of Vo1 is connected to Vo3 through the load current senseresistor R3. In this case, the gate drive of Q1 is a common grounddrive, which simplifies implementation.

In some embodiments wherein an integrated control and power circuit isused, the reference voltage is inside the IC chip and it cannot bechanged or controlled. The embodiment of FIG. 19 overcomes thislimitation. In FIG. 19, the ripple voltage of the main output (betweenVo1+ and Vo1−) is sensed at 120 and separated from the DC voltage at122, and added at 190 to the output voltage Vo3+ of the ripplecancellation converter 35, and the combined voltage is used as thefeedback voltage for the ripple cancellation converter. The outputvoltage of the ripple cancellation converter includes an AC voltagecomponent that is the same in magnitude and in opposite phase with theAC voltage component (i.e., ripple) of the main output (between Vo1+ andVo1−). As a result, the voltage across the load 100, in this case anLED, will be a substantially ripple-free DC voltage.

Variable Output Power

In some applications it might be desirable to vary (e.g., control) theoutput power of the converter. This can be achieved by controllingeither the output voltage or current, or by controlling the output powerbetween two different levels according to a variable duty cycle. Forexample, the two different powers levels might be switched between 0%and 100% of the output power.

For example, in some applications, such as LED lighting, dimming of theLED is required. The term “dimming” means that the light output of theLED is adjustable or variable. Dimming may be achieved by adjusting theaverage load current. Some applications may require that the LED lightis adjustable from 100% to 1%.

One method to achieve dimming is to reduce the regulated load current,as discussed with reference to FIGS. 14, 15, and 16. However, inpractical implementation, the current reference voltage change will betoo wide. For example, if the current sense resistor value is selectedso that at full load, the voltage across the current sense resistor is0.5 V, the voltage across the current sense resistor will be 5 mV whenthe required LED current is 1% of the full load. This voltage level willbe too low for the current error amplifier 60 (e.g., OpAmp3) to operateproperly. One way to avoid this problem is to add a DC offset to thecurrent sense voltage. An example of one way to implement this is shownin the embodiment of FIG. 16. In FIG. 16, a DC offset circuit includesan I_led_con signal and R11 and R12, wherein I_led_con may be anexternally generated LED light control signal. With this configuration,the voltage at pin6, I_led, can be calculated approximately as:I_led=I_led_con*R12/(R11+R12)+Iout*Rs  (23)

The current loop implemented by opamp3 forces the I_led voltage to bethe same as the reference voltage I_led_ref. When I_led_on is zero, orvery low,I_led=Iout*Rs  (24)and the LED current is high. When the voltage level of I_led_con isincreased, I_led_con* R12/(R11+R12) will increase, and the Iout*Rs willbe reduced, as the sum of these two signals is substantially equal toI_led_ref. Therefore, the LED current Iout will be reduced. With thisimplementation, the voltage at the input terminal (pin6) of the opampwill be maintained at a value suitable for proper operation, such as,for example, about 0.5 V, based on the above. For example, R11, R12 maybe selected such that when I_led_con is 10 V, the voltage across R12 is0.495V. The voltage across Rs will then be 0.005 V after feedbackcontrol. This effectively adjusts the LED current to about 1% of therated current. When I_led_con is 5 V, the voltage across R12 will beabout 0.25 V and the LED current will be about 50% of its rated current.

Another example of a dimming method is to turn off the ripplecancellation converter for a certain period of time, as shown in theembodiment of FIG. 20. For example, if the ripple cancellation converter35 is on all the time, the load current will be at its highest value andthe LED will be brightest. If the RCC is on for half the time, the loadcurrent will be half of the full load current. For preferred performancethe on/off frequency of the RCC should be selected to be higher than theline frequency and lower than the converter switching frequency. As anexample, the on/off frequency of the RCC may be 500 Hz to 2000 Hz,although other frequencies may be used as required for a given design orapplication.

One way to implement a control strategy for such an embodiment is to adda switch (e.g., MOSFET) to the ripple cancellation converter, as shownin the embodiment of FIG. 21. For example, as compared with theembodiment shown in FIG. 10, a switch S3 is added. During normaloperation, i.e., non-dimming operation, S3 is turned on all the time andthe circuit operation is substantially the same as before. Duringdimming operation, a dimming control signal, Vgs3, is applied to thegate of S3, as shown in the example of FIG. 22. The switching period, TsisTs_S3=Ton_S3+Toff_S3  (25)For proper operation the switching frequency of the dimming signalFs_S3=1/Ts_S3 is selected to be lower than the switching frequency ofthe buck converter and higher than the line frequency. For example, ifthe switching frequency of the buck converter is about 100 kHz to 1 MHz,Fs_S3 may be selected to be about 500 Hz to 2000 Hz. In this way, therewill be multiple of switching cycles within the Ton_S3 period. Duringdimming operation, when S3 is on (during the Ton_S3 period), theoperation of the converter is substantially the same as before. Duringthe Toff_S3 period, S1, S2, and S3 will all be off. The energy stored inL3 will be transferred to C3 in a very short period of time andsubstantially no current will flow through the Buck converter.Therefore, the current through the load will also be zero. The waveformof the load current is shown in the example of FIG. 22 (bottom). Theaverage LED current can be calculated as:I_led_avg=I_led_ref*Ton_S3/Ts_S3  (26)Therefore, by adjusting the ratio of Ton_S3/Ts_S3, the average LEDcurrent can be adjusted over a wide range, such that dimming isachieved.

Another reason to select the on/off frequency of S3 to be, e.g., severalhundred to several thousand Hz is that the human eye cannot detect thelight change in this frequency range.

In various embodiments switch S3 may be placed in series with switchSb2, or in series with inductor L3. These methods may applied toembodiments wherein the ripple cancellation converter is implementedusing other topologies, such as, for example, boost or buck-boostconverters, to achieve dimming operation.

Embodiments are further described by way of the following non-limitingexample.

EXAMPLE

Methods and circuits as described above were applied to a power supplyto investigate efficiency and current ripple reduction performance. Ablock diagram of the test setup is shown in FIG. 23, and Table 1provides details on the test equipment used. For this example, a Buckconverter was used for the ripple cancellation converter, and LEDs wereused as the load.

TABLE 1 Measurement Equipment AC power source Agilent 6813B DC voltagesource Agilent 6654A Multi-meter Fluke 4 digit multi-meter Load 22 LEDs,LR W5AM-HZJZ-1-Z (OSRAM Opto Semiconductors) Signal generator Agilent33250A Oscilloscope Tektronix TDS3034B Current probe Tektronix TCP202

Addition of the ripple cancellation converter substantially reduced ACripple in the output current. FIG. 24 shows the measured load currentripple, with 120V AC input and an LED voltage of about 50V DC and LED DCcurrent of about 0.7 A DC. It is noted that the load current ripple isabout 10 mA peak to peak, which is very small as compared with the DCcurrent of 0.7 A. The AC ripple at the main output (between Vo+ and Vo−of FIG. 23) was about 3 V peak to peak. FIG. 25 shows the input ACcurrent waveform at the same conditions. It can be seen that the ACcurrent is a sinusoidal waveform, which indicates high power factor isachieved. The power factor was measured to be 0.99.

Equivalents

Those skilled in the art will recognize or be able to ascertain variantsof the embodiments described herein. Such variants are within the scopeof the invention and are covered by the appended claims.

The invention claimed is:
 1. A circuit for use with a power supply thatprovides a first output including a first DC voltage with a first ACvoltage ripple or a first DC current with a first AC current ripple, thecircuit comprising: a ripple cancellation converter that: (i) provides asecond AC voltage ripple equal in magnitude and opposite in phase to thefirst AC voltage ripple, and is connected in series with the firstoutput, such that the first AC voltage ripple is substantiallycancelled; or (ii) provides a second AC current ripple equal inmagnitude and opposite in phase to the first AC current ripple, and isconnected in parallel with the first output, such that the first ACcurrent ripple is substantially cancelled; and a controller that sensesa peak to peak value of the first AC voltage ripple or the first ACcurrent ripple and controls the ripple cancellation converter tominimize a DC offset of the second AC voltage ripple or the second ACcurrent ripple; wherein substantially ripple-free DC output power isprovided.
 2. The circuit of claim 1, wherein the ripple cancellationconverter comprises a buck converter, a boost converter, a buck-boostconverter, or a full bridge converter.
 3. The circuit of claim 1,further comprising a power factor correction circuit.
 4. The circuit ofclaim 3, wherein operation of the power factor correction circuit isbased on a feedback signal derived from a load current.
 5. The circuitof claim 1, wherein the substantially ripple-free DC output power isvariable.
 6. The circuit of claim 1, for use with an AC-DC power supply.7. The circuit of claim 1, wherein the power supply comprises a flybackconverter, an isolated boost converter, a buck-boost converter, a buckconverter, or a boost converter.
 8. The circuit of claim 1, wherein aload comprises an LED.
 9. A method for providing substantiallyripple-free DC power from a first output power comprising a first DCvoltage with a first AC voltage ripple or a first DC current with afirst AC current ripple, the method comprising: sensing a peak to peakvalue of the first AC voltage ripple or the first AC current ripple; (i)providing a second AC voltage ripple equal in magnitude and opposite inphase to the first AC voltage ripple; minimizing a DC offset of thesecond AC voltage ripple based on a sensed value of the first AC voltageripple; connecting the second AC voltage ripple in series with the firstoutput power, such that the first AC voltage ripple is substantiallycancelled; or (ii) providing a second AC current ripple equal inmagnitude and opposite in phase to the first AC current ripple;minimizing a DC offset of the second AC current ripple based on a sensedvalue of the first AC current ripple; connecting the second AC currentripple in parallel with the first output, such that the first AC currentripple is substantially cancelled; wherein substantially ripple-free DCoutput power is provided.
 10. The method of claim 9, including using abuck converter, a boost converter, a buck-boost converter, or a failbridge converter to provide the second AC voltage ripple or to providethe second AC current ripple.
 11. The method of claim 9, furthercomprising adjusting a power factor of a circuit used to provide thefirst DC power.
 12. The method of claim 11, comprising adjusting thepower factor based on a feedback signal derived from a load current. 13.The method of claim 9, comprising using an AC-DC power supply to providethe main output power.
 14. The method of claim 13, including using aflyback converter, an isolated boost converter, a buck-boost converter,a buck converter, or a boost converter for the ACDC power supply. 15.The method of claim 9, comprising connecting the substantiallyripple-free DC output power to a load, wherein the load comprises anLED.
 16. The circuit of claim 1, wherein: (i) the DC offset of thesecond AC voltage ripple is equal to (a) a peak value of the AC voltageripple, or (b) (0.5+K) times the peak to peak value of the AC voltageripple; or (ii) the DC offset of the second AC current ripple is equalto (a) a peak value of the AC current ripple, or (b) (0.5+K) times thepeak to peak value of the AC voltage ripple; wherein K is from 0.1 to1.0.
 17. The circuit of claim 1, wherein the ripple cancellationconverter comprises a full bridge converter.
 18. The circuit of claim17, wherein: (i) the DC offset of the second AC voltage ripple is lessthan a peak value of the first AC voltage ripple; or (ii) the DC offsetof the second AC current ripple is less than a peak value of the firstAC current ripple.
 19. The method of claim 9, wherein: (i) the DC offsetof the second AC voltage ripple is equal to (a) a peak value of the ACvoltage ripple, or (b) (0.5+K) times the peak to peak value of the ACvoltage ripple; or (ii) the DC offset of the second AC current ripple isequal to (a) a peak value of the AC current ripple, or (b) (0.5+K) timesthe peak to peak value of the AC voltage ripple; wherein K is from 0.1to 1.0.
 20. The method of claim 9, including using a full bridgeconverter to provide the second AC voltage ripple or to provide thesecond AC current ripple.
 21. The method of claim 20, wherein minimizingcomprises: (i) setting the DC offset of the second AC voltage ripple tobe less than a peak value of the first AC voltage ripple; or (ii)setting the DC offset of the second AC current ripple to be less than apeak value of the first AC current ripple.